With the rise in natural gas prices attention is being focused on new sources of energy. It has been known for many years that there is a large quantity of natural gas associated with underground coal beds. Due to the porous nature of coal, many times the volume of gas per cubic foot of solid material is more readily available when compared to gas found in rocky formations. The gas is under high pressure, and is associated with large quantities of water which also permeate the coal bed.
The most common method for extracting this coal-bed methane is to drill a well into the coal and pump out the water, thus lowering the pressure on the gas. As the pressure on the bed is lowered, gas migrates to the well and is removed, separated from the water, and processed for shipment. The pumps are electrically driven and pumping is regulated by using variable-frequency drives (VFDs) to control motor speed. These VFDs draw harmonic currents from the utility electrical system, thus providing an opportunity for the application of harmonic mitigating technologies.
Harmonics in an electrical distribution system are caused by the types of loads connected to the system. Modern electronic equipment and controls, because they draw current in a non-linear fashion, causing harmonic currents to flow throughout the system. The extra harmonic currents may result in overloading or failure of system components.
The VFDs powering pumps used for coal-bed methane production, like all other VFDs, draw these harmonic currents. Most of the pumping sites are remote to civilization and most do not have power already available. Utilities have been forced to extend power lines many miles for fairly small loads; not a cost effective practice. To save money on running long lines, the utility companies usually provide only single-phase 480 or 240 volt power, thus reducing line costs. However, there is a major problem with this method of power distribution.
There are few or no VFDs made over about 10 hp that are designed to run on single-phase power. It is much more cost effective to install a 3-phase drive and power it from a single-phase line. The drive must be de-rated, such that it is capable of handling only about 57% of the drive's nameplate horsepower. Common practice is to de-rate a drive to about 50% nameplate when it is single phased. (i.e., to run a 10 hp motor, a 20 hp drive is required.) This means that the single-phase line must supply about twice the current that a 3-phase line would supply to power the motor. This also means that the harmonic currents drawn by the drive are not in the 3-phase spectrum (5 & 7, 11 & 13, etc.), but are instead the single phase spectrum consisting primarily of the 3rd harmonic. The 3rd harmonic current drawn by a drive may optionally be equal to or greater than the fundamental (60 Hz in the US) current.
These extra harmonic currents must be transmitted through long distribution lines to the drive, causing significant power or energy losses and voltage drop. Further, when the various lines to multiple drives are combined back onto the 3-phase system, the 3rd harmonic currents will add into the distribution system neutral, back to the substation, causing further power or energy losses.
Again, since this related system is not very robust, the high 3rd harmonic currents cause problems at the substation level. Problems include, but are not limited to, reduced capacity on an already weak system and neutral and transformer overloading and overheating.
This harmonic problem has been observed wherever single-phase drives are used to support coal bed gas production, and the number of drives causing the problem is increasing rapidly. At least one utility company already has over 2,000 wells connected and has plans for over 500 more per year in the near future. An improved Harmonic Suppression System, designed specifically for a remote site and/or coal-bed methane pumping applications is disclosed herein. The present invention improves upon the invention disclosed at U.S. Pat. No. 6,636,405, with the present invention having particular application to the remote field locations described above. Testing has shown that this improved Harmonic Suppression System design is successful in reducing the 3rd harmonic problem.